1. Field of the Invention
The present invention relates in general to a rising and falling weir made of a flexible membrane and, more particularly, to a pneumatically inflatable weir which is equipped with improved drain relieving means.
2. Description of the Prior Art
A rising and falling weir made of a flexible membrane is anchored at a crest of a concrete structure in a body of water to control water flow by inflating the weir to raise it or deflating it to lower it.
The basic structure of an inflatable weir includes an envelope of rubberized fabric or the like attached to a bed portion of the crest. A pressure medium, such as air and/or water, is charged into the envelope to inflate the weir to an upstanding position. The pressure medium is discharged from the envelope to shrink it to a folded position (e.g., Japanese Unexamined Patent Publication Nos. 11702/1965 and 2371/1969).
With reference to FIG. 1, when an inflatable weir according to the prior art is attached to the crest of a high dam made of concrete 1 to obtain high water level damming performance, water 4 accumulates inside of an envelope 2 at the bottom portion thereof, because this bottom portion is located below the level of a water charging/discharging port 3 used to inflate/deflate the envelope 2. Water vapor in the air injected into the envelope 2, via a charging/discharging pipe 5, is cooled by the body of water on which the weir is used. The steam condenses into water 4. As a result, water 4 remains in the envelope 2 when it is deflated. This remaining water 4 prevents the weir from completely shrinking to its folded position on the crest and prevents dammed water from smoothly falling over the crest after deflation of the envelope 2.
In such a conventional weir, since the water charging/discharging port 3 is located above the level of the lowest portion of the envelope 2 which is the bottom of the envelope 2 in its folded position, the remaining water 4 cannot be completely discharged.